{"gene":"PPP2R5D","run_date":"2026-06-10T06:43:35","timeline":{"discoveries":[{"year":2007,"finding":"PKA phosphorylates the B56δ (PPP2R5D) regulatory subunit of PP2A at Ser-566, which increases the overall phosphatase activity of the PP2A-B56δ/A/C heterotrimer in vitro and in vivo. The B56δ-containing PP2A complex dephosphorylates DARPP-32 at Thr-75, thereby mediating cAMP/PKA-dependent regulation of dopaminergic signaling in striatal neurons.","method":"In vitro kinase assay, site-directed mutagenesis (Ser-566 phosphorylation sites), cotransfection in HEK293 cells, striatal slice phosphorylation assays","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 1 / Strong — in vitro reconstitution with mutagenesis, validated in vivo in striatal slices, multiple orthogonal methods in single rigorous study","pmids":["17301223"],"is_preprint":false},{"year":2006,"finding":"PP2A/B56δ phosphatase complex dephosphorylates Cdc25 at T138 (Xenopus numbering), a site distinct from the inhibitory Ser287, thereby reducing Cdc25 affinity for 14-3-3 proteins and enabling 14-3-3 release to promote mitotic entry. DNA-responsive checkpoints activate PP2A/B56δ complexes, identifying B56δ as a central checkpoint effector.","method":"Xenopus cell-free extract and cell-based assays, immunoprecipitation, phosphorylation site mapping, dominant-negative and rescue experiments","journal":"Cell","confidence":"High","confidence_rationale":"Tier 1-2 / Strong — multiple orthogonal biochemical and cell-based assays, published in high-impact venue with rigorous controls","pmids":["17110335"],"is_preprint":false},{"year":2011,"finding":"In Ppp2r5d knockout mice, tau becomes progressively hyperphosphorylated at pathological epitopes in restricted brain areas. The mechanism is indirect: PP2A-B56δ (PP2A-T61δ) dephosphorylates p35 (the CDK5 activator), and its absence leads to p35 hyperphosphorylation and degradation, thereby reducing CDK5 activity. Loss of CDK5 activity results in decreased phosphorylation of GSK3β at Ser-9, increasing GSK3β activity and causing tau hyperphosphorylation.","method":"Ppp2r5d knockout mouse model, in vitro dephosphorylation assays, immunohistochemistry, kinase activity assays, behavioral testing","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 1-2 / Strong — genetic KO with defined biochemical mechanism, in vitro phosphatase assay, multiple substrates tested","pmids":["21482799"],"is_preprint":false},{"year":2015,"finding":"De novo missense mutations in PPP2R5D within a conserved acidic loop render mutant B56δ deficient in binding the PP2A A and C subunits, uncoupling it from phosphatase activity. This dominant-negative effect results in hyperphosphorylation of the B56δ-regulated substrate GSK3β in cells overexpressing mutant subunits.","method":"Co-immunoprecipitation for A/C subunit binding, phosphatase activity assays, overexpression of mutant subunits in cells, GSK3β phosphorylation immunoblot","journal":"The Journal of clinical investigation","confidence":"High","confidence_rationale":"Tier 2 / Strong — reciprocal Co-IP, activity assays, substrate phosphorylation readout, replicated across multiple mutations in 16 patients","pmids":["26168268"],"is_preprint":false},{"year":2014,"finding":"B56δ is the specific PP2A regulatory subunit mediating dephosphorylation of Bcl-2 at Ser70. Peroxynitrite-mediated nitration of B56δ at Tyr289 inhibits recruitment of the PP2A catalytic core (A and C subunits) to the B56δ complex while preserving B56δ binding to phospho-Ser70-Bcl-2, thereby preventing Bcl-2 dephosphorylation and stabilizing its antiapoptotic activity.","method":"Genetic knockdown of SOD1, co-immunoprecipitation, site-specific mutagenesis of B56δ Tyr289, PP2A holoenzyme assembly assays, Bcl-2 phosphorylation immunoblot","journal":"Blood","confidence":"High","confidence_rationale":"Tier 1-2 / Strong — mutagenesis of nitration site, reciprocal Co-IP for holoenzyme assembly, validated in primary lymphoma cells","pmids":["25082878"],"is_preprint":false},{"year":2011,"finding":"Protein kinase C phosphorylates a key regulatory site in B56δ, activating the B56δ-containing PP2A heterotrimer, which then dephosphorylates Ser40 of tyrosine hydroxylase. RNAi knockdown of B56δ in N27 cells increases dopamine synthesis, confirming that PP2A-B56δ-mediated TH dephosphorylation reduces TH activity.","method":"In vitro kinase assay, RNAi knockdown, dopamine synthesis measurement, phosphorylation assays in cell lines","journal":"PloS one","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — RNAi KD with functional readout (dopamine synthesis), kinase assay, but single lab","pmids":["22046270"],"is_preprint":false},{"year":2010,"finding":"PKA phosphorylates PPP2R5D at Ser-53, Ser-68, Ser-81, and Ser-566 in vitro, but Ser-566 is the dominant in vivo phosphorylation site upon forskolin stimulation. Phosphorylation of PPP2R5D by PKA reduces the apparent Km of PP2A holoenzyme from 11.25 μM to 1.175 μM, increasing catalytic efficiency. Phosphorylation also decreases inhibitory Tyr-307 phosphorylation on the PP2A catalytic C subunit.","method":"In vitro kinase assay, site-directed mutagenesis of phosphorylation sites, kinetic analysis in HEK293 cells with forskolin, phospho-immunoblot","journal":"BMB reports","confidence":"Medium","confidence_rationale":"Tier 1 / Weak — in vitro kinase assay and kinetics with mutagenesis, but single lab and limited validation","pmids":["20423611"],"is_preprint":false},{"year":2017,"finding":"β-Adrenergic receptor stimulation induces PKA-mediated phosphorylation of B56δ at Ser573 in adult rat ventricular cardiomyocytes. A non-phosphorylatable S573A mutant B56δ fails to increase PP2A catalytic activity in response to isoprenaline, demonstrating that Ser573 phosphorylation is required for β-AR-stimulated PP2A activation in cardiomyocytes.","method":"Phosphate-affinity SDS-PAGE, adenoviral transduction with WT and S573A mutant B56δ-GFP, co-immunoprecipitation with A/C subunits, PP2A activity assay, phosphoproteomics immunoblotting in ARVMs","journal":"Journal of molecular and cellular cardiology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — mutagenesis in relevant cell type, co-IP, activity assay, single lab with two orthogonal methods","pmids":["29294329"],"is_preprint":false},{"year":2019,"finding":"PPM1G (a PPM-family phosphatase) forms a novel holoenzyme complex with B56δ (PPP2R5D). B56δ promotes relocalization of PPM1G from the nucleus to the cytoplasm. The PPM1G-B56δ complex dephosphorylates α-catenin at Ser641 in the cytoplasm, which is required for proper adherens junction assembly and prevention of aberrant cell migration.","method":"Co-immunoprecipitation, subcellular fractionation/live imaging for relocalization, in vitro phosphatase assay on α-catenin, knockdown/rescue experiments for cell migration and junction assembly","journal":"EMBO reports","confidence":"High","confidence_rationale":"Tier 1-2 / Strong — Co-IP of novel complex, substrate identified with in vitro dephosphorylation, functional consequence (adherens junctions, migration) validated","pmids":["31432583"],"is_preprint":false},{"year":2015,"finding":"B56δ-containing PP2A directly binds C/EBPβ and dephosphorylates it during early adipogenesis. This dephosphorylation is required for C/EBPβ degradation, which allows subsequent expression of PPARγ and C/EBPα and completion of adipogenesis.","method":"Co-immunoprecipitation of B56δ with C/EBPβ, okadaic acid inhibition, knockdown of specific B56 subunits, adipogenesis induction assay, immunoblotting for adipogenic transcription factors","journal":"Biochimica et biophysica acta","confidence":"Medium","confidence_rationale":"Tier 2-3 / Moderate — direct binding shown by Co-IP, functional knockdown with defined transcription factor readout, single lab","pmids":["25152162"],"is_preprint":false},{"year":2015,"finding":"B56δ-containing PP2A dephosphorylates eNOS at Ser116. Aphidicolin-induced DNA damage increases B56δ-Ser566 phosphorylation, activating PP2A-B56δ, which dephosphorylates eNOS-Ser116 and contributes to NO release in endothelial cells. Dominant-negative B56δ blocks both Ser116 dephosphorylation and NO production.","method":"Dominant-negative B56δ overexpression, okadaic acid inhibition, phospho-immunoblot for eNOS-Ser116 and B56δ-Ser566, NO measurement in bovine aortic endothelial cells","journal":"Nitric oxide : biology and chemistry","confidence":"Medium","confidence_rationale":"Tier 2-3 / Moderate — dominant-negative mutation with two phosphorylation site readouts and functional NO assay, single lab","pmids":["26255574"],"is_preprint":false},{"year":2021,"finding":"A CRISPR-base-edited E420K heterozygous variant of PPP2R5D in HEK293 cells reveals a direct interaction between PPP2R5D and AKT, leading to constitutively active AKT-mTOR signaling, increased cell size, and uncoordinated cellular growth. Rapamycin reduces cell size and RPS6 hyperphosphorylation in E420K variant cells.","method":"CRISPR single-base editing to introduce E420K variant, quantitative phosphoproteomics (TMT-LC-MS3), co-immunoprecipitation of PPP2R5D-AKT, rapamycin treatment, cell size measurement","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1-2 / Strong — precise endogenous variant introduced by CRISPR, unbiased phosphoproteomics plus orthogonal Co-IP, functional cell size readout, single lab with multiple methods","pmids":["33482199"],"is_preprint":false},{"year":2017,"finding":"Loss of PP2A-B56δ in knockout mice leads to spontaneous hepatocellular carcinoma, associated with c-Myc Ser62 hyperphosphorylation and GSK3β Ser9 hyperphosphorylation in non-cancerous B56δ-null livers, indicating that B56δ-driven GSK3β inactivation controls c-Myc activity as a tumor suppressive mechanism.","method":"Ppp2r5d knockout mouse model, targeted immunoblotting, immunohistochemistry, RNA sequencing, phospho-specific antibodies for c-Myc Ser62 and GSK3β Ser9","journal":"Oncogene","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — genetic KO with phosphoprotein readouts and transcriptomic support, single lab","pmids":["28967903"],"is_preprint":false},{"year":2022,"finding":"PPP2R5D interacts specifically with HCV NS5B RNA-dependent RNA polymerase (but not HCV Core or NS3), colocalizes with NS5B in the endoplasmic reticulum, and is required for HCV replication. Knockout of PPP2R5D abolishes HCV infection in Huh7.5 cells, and re-expression restores infection.","method":"Co-immunoprecipitation, colocalization by imaging, PPP2R5D knockout and knockdown with complementation, HCV replicon replication assay","journal":"Virology journal","confidence":"Medium","confidence_rationale":"Tier 2-3 / Moderate — reciprocal Co-IP plus KO/complementation with viral replication readout, single lab","pmids":["35836293"],"is_preprint":false},{"year":2023,"finding":"Cryo-EM structures of the PP2A-B56δ holoenzyme reveal that long intrinsically disordered regions (IDRs) at B56δ N- and C-termini fold against each other and the holoenzyme core, forming a closed latent conformation with dual autoinhibition of the phosphatase active site and the substrate-binding groove. This interface spans >190 Å, harbors activation phosphorylation sites and essentially all ID-associated mutation residues. ID mutations increase holoenzyme activity and alter phosphorylation rates; severe variants significantly increase mitotic duration and error rates.","method":"Single-particle cryo-EM structure determination, in vitro phosphorylation assays, mitotic duration and error rate measurements in cells with disease variants","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 1 / Strong — cryo-EM structural determination combined with functional phosphorylation and cell-biological assays for disease mutations","pmids":["38150499"],"is_preprint":false},{"year":2023,"finding":"Quantum mechanical calculations on the PP2A(PPP2R5D)/phosphoserine system indicate that bidentate Arg89-substrate binding is critical for optimal catalytic function, yielding ΔH‡ = +15.5 kcal/mol vs +18.8 kcal/mol when Arg89 is engaged in a salt bridge with B56δ Glu198. The pathogenic E198K mutation replaces the acidic Glu198 with a positively charged Lys, disrupting this salt bridge and altering the catalytic mechanism.","method":"Quantum-based hybrid ONIOM(UB3LYP/6-31G(d):UPM7) computational modeling of 39-residue active-site models","journal":"Frontiers in cell and developmental biology","confidence":"Low","confidence_rationale":"Tier 4 / Weak — computational modeling only, no experimental validation in this study","pmids":["37377738"],"is_preprint":false},{"year":2023,"finding":"PPP2R5D/PP2A-B56δ interacts with liprin-α1 through a canonical short linear interaction motif (SLiM) in liprin-α1's N-terminal dimerization domain. Loss of this interaction (SLiM mutation or PPP2R5D KO) results in increased liprin-α1 phosphorylation at Ser763 and promotes liprin-α1 liquid-liquid phase separation (LLPS). The E420K disease variant of PPP2R5D compromises suppression of liprin-α1 LLPS.","method":"MS-based interactomics, co-immunoprecipitation, SLiM mutagenesis, PPP2R5D KO cells, GFP-liprin-α1 LLPS imaging, phospho-mass spectrometry","journal":"bioRxiv","confidence":"Medium","confidence_rationale":"Tier 2-3 / Moderate — MS interactomics with SLiM mutagenesis, KO cell validation, LLPS imaging; preprint","pmids":["38948786"],"is_preprint":true},{"year":2025,"finding":"PPP2R5D-PP2A holoenzyme inhibits liprin-α1 LLPS by dephosphorylating liprin-α1 at multiple Ser/Thr sites including Ser763. The phospho-mimetic S763E mutant is sufficient to drive liprin-α1 LLPS. The E420K PPP2R5D disease variant increases liprin-α1 Ser763 phosphorylation and promotes LLPS. The interaction also promotes liprin-α1/β1 heterodimerization, which opposes LLPS.","method":"Mass spectrometry phospho-analysis, phospho-specific antibody validation, SLiM mutagenesis, PPP2R5D KO and E420K knock-in cells, LLPS imaging, liprin-α1/β1 co-immunoprecipitation","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1-2 / Strong — multiple orthogonal methods (phospho-MS with novel antibody validation, KO and knock-in cells, SLiM mutagenesis, LLPS functional readout) in single rigorous study","pmids":["40484382"],"is_preprint":false},{"year":2022,"finding":"Functional characterization of PPP2R5D missense variants shows that pathogenic variants cause impaired PP2A A/C-subunit binding, decreased SLiM-dependent substrate binding, or both. The most severe clinical phenotypes associate with variants that completely lose one of these binding properties while retaining the other, supporting a dominant-negative disease mechanism. The p.Glu198Lys variant shows the highest C-binding defect.","method":"Co-immunoprecipitation for A/C subunit binding, SLiM-dependent substrate binding assays, correlation with clinical phenotype severity in 76 patients","journal":"Journal of medical genetics","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — Co-IP binding assays for multiple variants with phenotypic correlation, single lab","pmids":["36216457"],"is_preprint":false},{"year":2021,"finding":"Double knockout of Ppp2r5d (B56δ) and Ppp2r5c (B56γ) in mice causes arrest of fetal development around E12 and results in a single cardiac outflow vessel instead of separate aorta and pulmonary artery, demonstrating a genetic interaction between B56δ and B56γ that is required for heart development. Neither single knockout alone is lethal.","method":"CRISPR/Cas9n knockout mouse generation, genetic epistasis (double KO), embryonic lethal phenotype analysis, cardiac anatomy assessment","journal":"FASEB bioAdvances","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — genetic epistasis in mouse KO model with defined developmental phenotype, single lab","pmids":["35415460"],"is_preprint":false},{"year":2023,"finding":"Quantitative phosphoproteomics of CRISPR-edited E198K and E420K PPP2R5D variant HEK293 cells reveals hyperphosphorylation of RPS6 as a shared signaling alteration, mediated through converging mTORC1/p70S6K activation. E420K shows AKT-dependent mTORC1 activation while E198K shows AKT-independent ERK-dependent activation. Rapamycin and the S6K inhibitor LY2584702 suppress RPS6 hyperphosphorylation.","method":"CRISPR-PRIME editing for E198K variant, global quantitative proteomics and phosphoproteomics, rapamycin and kinase inhibitor treatments, immunoblotting","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — global phosphoproteomics on endogenous CRISPR-edited variants, pharmacological validation, single lab","pmids":["37572851"],"is_preprint":false}],"current_model":"PPP2R5D encodes B56δ, a regulatory subunit that assembles with PP2A scaffold (A) and catalytic (C) subunits to form a heterotrimer whose substrate specificity, subcellular localization, and activity are governed by B56δ; PKA and PKC phosphorylate B56δ at Ser566/Ser573 to increase PP2A catalytic efficiency (lowering Km), while peroxynitrite nitration of B56δ Tyr289 disassembles the holoenzyme; structurally, long intrinsically disordered arms of B56δ fold against the holoenzyme core to create dual autoinhibition that is relieved by activating phosphorylation; established substrates dephosphorylated by PP2A-B56δ include DARPP-32 (Thr75), Cdc25 (T138), p35/CDK5 activator, GSK3β (controlling tau phosphorylation and c-Myc stability), Bcl-2 (Ser70), tyrosine hydroxylase (Ser40), C/EBPβ (early adipogenesis), eNOS (Ser116), α-catenin (Ser641 via PPM1G-B56δ), AKT, and liprin-α1 (Ser763); disease-causing de novo missense mutations disrupt A/C-subunit binding or SLiM-dependent substrate recruitment in a dominant-negative manner, leading to hyperactivation of AKT-mTOR-RPS6 and GSK3β-c-Myc signaling cascades underlying intellectual disability, macrocephaly, and tumorigenesis."},"narrative":{"mechanistic_narrative":"PPP2R5D encodes B56δ, a substrate-specifying regulatory subunit that assembles with the PP2A scaffold (A) and catalytic (C) subunits into a heterotrimeric holoenzyme directing protein dephosphorylation across neuronal signaling, cell-cycle control, and developmental programs [PMID:17301223, PMID:26168268]. Holoenzyme activity is gated by upstream phosphorylation: PKA and PKC phosphorylate B56δ at Ser566/Ser573 to raise catalytic efficiency by lowering the apparent Km and reducing inhibitory phosphorylation of the C subunit [PMID:17301223, PMID:20423611, PMID:29294329], a regulatory logic explained structurally by intrinsically disordered B56δ termini that fold against the holoenzyme core to impose dual autoinhibition of both the active site and the substrate-binding groove, relieved by these activating phosphorylation events [PMID:38150499]. Through this holoenzyme, B56δ dephosphorylates a wide substrate set with distinct physiological outputs—DARPP-32 Thr75 in dopaminergic signaling [PMID:17301223], Cdc25 to license mitotic entry [PMID:17110335], the CDK5 activator p35 governing GSK3β-dependent tau phosphorylation [PMID:21482799], tyrosine hydroxylase Ser40 controlling dopamine synthesis [PMID:22046270], Bcl-2 Ser70 [PMID:25082878], C/EBPβ during adipogenesis [PMID:25152162], eNOS Ser116 [PMID:26255574], and liprin-α1 Ser763, where dephosphorylation suppresses liquid-liquid phase separation [PMID:38948786, PMID:40484382]. B56δ also nucleates non-canonical complexes, partnering with PPM1G to relocalize it and dephosphorylate α-catenin Ser641 at adherens junctions [PMID:31432583]. Loss of B56δ inactivates GSK3β control of c-Myc, driving spontaneous hepatocellular carcinoma in knockout mice [PMID:28967903], and B56δ is genetically required with B56γ for cardiac outflow tract development [PMID:35415460]. De novo missense mutations in PPP2R5D cause an intellectual disability and macrocephaly syndrome through a dominant-negative mechanism, impairing A/C-subunit binding and/or SLiM-dependent substrate recruitment and producing hyperactivation of AKT-mTOR-RPS6 and GSK3β-c-Myc signaling [PMID:26168268, PMID:33482199, PMID:36216457, PMID:37572851].","teleology":[{"year":2006,"claim":"Established B56δ-PP2A as a cell-cycle checkpoint effector by identifying how it relieves 14-3-3 sequestration of Cdc25 to control mitotic entry.","evidence":"Xenopus cell-free extract and cell-based assays with phosphosite mapping, dominant-negative and rescue experiments","pmids":["17110335"],"confidence":"High","gaps":["Did not resolve how DNA-damage checkpoints physically activate the B56δ holoenzyme","Human in vivo relevance not addressed"]},{"year":2007,"claim":"Showed that B56δ phosphorylation is an activating switch, linking PKA phosphorylation of Ser566 to increased phosphatase activity and DARPP-32 dephosphorylation in neurons.","evidence":"In vitro kinase assay, site-directed mutagenesis, HEK293 cotransfection, striatal slice phosphorylation assays","pmids":["17301223"],"confidence":"High","gaps":["Structural basis of phosphorylation-induced activation unknown at this stage","Full substrate repertoire untested"]},{"year":2010,"claim":"Quantified the activating mechanism, showing PKA phosphorylation of Ser566 lowers holoenzyme Km roughly tenfold and decreases inhibitory C-subunit Tyr307 phosphorylation.","evidence":"In vitro kinase assay, mutagenesis, kinetic analysis with forskolin in HEK293 cells","pmids":["20423611"],"confidence":"Medium","gaps":["Single lab, limited validation","Roles of additional sites Ser53/68/81 in vivo not resolved"]},{"year":2011,"claim":"Connected B56δ loss to neurodegenerative tau pathology via an indirect p35/CDK5/GSK3β cascade, and to dopamine synthesis control via tyrosine hydroxylase Ser40.","evidence":"Ppp2r5d knockout mouse with in vitro dephosphorylation assays; PKC kinase assay with RNAi and dopamine measurement in N27 cells","pmids":["21482799","22046270"],"confidence":"High","gaps":["Direct versus indirect substrate relationships partly inferred","TH study from single lab"]},{"year":2014,"claim":"Revealed that post-translational modification can selectively disassemble the holoenzyme, with peroxynitrite nitration of B56δ Tyr289 blocking A/C recruitment while sparing substrate binding to stabilize Bcl-2.","evidence":"SOD1 knockdown, Tyr289 mutagenesis, holoenzyme assembly Co-IP, Bcl-2 phospho-immunoblot in lymphoma cells","pmids":["25082878"],"confidence":"High","gaps":["Generality of nitration regulation across tissues unknown"]},{"year":2015,"claim":"Extended the substrate landscape to metabolic and vascular programs through C/EBPβ dephosphorylation in adipogenesis and eNOS Ser116 dephosphorylation in endothelial NO release.","evidence":"Co-IP, knockdown, adipogenesis assays; dominant-negative B56δ, phospho-immunoblot and NO measurement in endothelial cells","pmids":["25152162","26255574"],"confidence":"Medium","gaps":["Both single-lab studies","Direct holoenzyme-substrate engagement not structurally defined"]},{"year":2015,"claim":"Defined the dominant-negative disease mechanism, showing de novo mutations in a conserved acidic loop uncouple B56δ from A/C binding and cause GSK3β hyperphosphorylation.","evidence":"Reciprocal Co-IP, phosphatase activity assays, mutant overexpression, GSK3β immunoblot across 16 patients","pmids":["26168268"],"confidence":"High","gaps":["Endogenous-level consequences not yet tested","Downstream signaling network not mapped"]},{"year":2017,"claim":"Tied B56δ to tumor suppression in vivo, with knockout livers developing hepatocellular carcinoma alongside GSK3β and c-Myc Ser62 hyperphosphorylation; also confirmed Ser573 as a β-adrenergic activation site in cardiomyocytes.","evidence":"Ppp2r5d KO mouse with phospho-immunoblot, IHC, RNA-seq; phosphate-affinity gels and S573A mutant in adult rat ventricular myocytes","pmids":["28967903","29294329"],"confidence":"Medium","gaps":["Single-lab studies","Causal chain from GSK3β to tumorigenesis not fully dissected"]},{"year":2019,"claim":"Uncovered a non-canonical role, in which B56δ partners with PPM1G to relocalize it and dephosphorylate α-catenin Ser641 for adherens junction assembly.","evidence":"Co-IP, subcellular fractionation/live imaging, in vitro phosphatase assay, knockdown/rescue migration assays","pmids":["31432583"],"confidence":"High","gaps":["Stoichiometry and structural basis of the PPM1G-B56δ complex unknown","Whether PP2A C subunit participates unclear"]},{"year":2021,"claim":"Resolved how disease variants drive growth signaling using endogenous CRISPR-edited cells, linking E420K B56δ to an AKT interaction and constitutive AKT-mTOR-RPS6 activation; double KO with B56γ showed an essential role in heart development.","evidence":"CRISPR base editing of E420K, phosphoproteomics, PPP2R5D-AKT Co-IP, rapamycin rescue; CRISPR/Cas9n double-KO mouse with cardiac anatomy analysis","pmids":["33482199","35415460"],"confidence":"High","gaps":["Whether AKT is a direct dephosphorylation substrate not fully established","Redundancy logic between B56δ and B56γ not molecularly defined"]},{"year":2022,"claim":"Systematically mapped variant biochemistry, separating A/C-binding defects from SLiM-dependent substrate-binding defects and correlating them with phenotype severity; also identified a host-factor role in HCV replication.","evidence":"Co-IP binding assays across variants with clinical correlation in 76 patients; Co-IP, colocalization and KO/complementation HCV replication assays","pmids":["36216457","35836293"],"confidence":"Medium","gaps":["Mechanism by which NS5B exploits B56δ unknown","Binding-assay severity correlation is associative"]},{"year":2023,"claim":"Provided the structural and mechanistic basis of regulation, showing disordered B56δ arms create dual autoinhibition spanning activation sites and disease residues, with variants increasing activity and mitotic errors; computational modeling implicated an Arg89-Glu198 salt bridge disrupted by E198K.","evidence":"Cryo-EM structure determination with in vitro phosphorylation and mitotic error assays; QM/ONIOM active-site modeling","pmids":["38150499","37377738"],"confidence":"High","gaps":["QM modeling lacks experimental validation (Low confidence)","How specific variants quantitatively shift substrate selectivity not fully resolved"]},{"year":2025,"claim":"Established liprin-α1 as a SLiM-recruited substrate whose Ser763 dephosphorylation suppresses liquid-liquid phase separation and promotes heterodimerization, with E420K B56δ failing to restrain this LLPS.","evidence":"MS interactomics, phospho-MS with antibody validation, SLiM mutagenesis, KO and E420K knock-in cells, LLPS imaging, Co-IP","pmids":["40484382","38948786"],"confidence":"High","gaps":["Physiological consequence of liprin-α1 LLPS dysregulation in patient neurons not shown","Whether LLPS control generalizes to other B56δ substrates unknown"]},{"year":null,"claim":"It remains unresolved how individual disease variants convert B56δ's dual-autoinhibited holoenzyme into specific gain- versus loss-of-function outputs on distinct substrates, and how these reconcile into the unified ID-macrocephaly-tumor phenotype.","evidence":"","pmids":[],"confidence":"Medium","gaps":["No structure of disease-variant holoenzyme bound to substrate","Tissue-specific substrate selectivity not mapped","Direct versus indirect status of several substrates (AKT) unconfirmed"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0140096","term_label":"catalytic activity, acting on a protein","supporting_discovery_ids":[0,1,2,4,8,9,10,17]},{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[0,3,14,18]},{"term_id":"GO:0060090","term_label":"molecular adaptor activity","supporting_discovery_ids":[8,16,17]}],"localization":[{"term_id":"GO:0005829","term_label":"cytosol","supporting_discovery_ids":[8]},{"term_id":"GO:0005634","term_label":"nucleus","supporting_discovery_ids":[8]},{"term_id":"GO:0005783","term_label":"endoplasmic reticulum","supporting_discovery_ids":[13]}],"pathway":[{"term_id":"R-HSA-1640170","term_label":"Cell Cycle","supporting_discovery_ids":[1,14]},{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[0,11,20]},{"term_id":"R-HSA-392499","term_label":"Metabolism of proteins","supporting_discovery_ids":[0,6,7]},{"term_id":"R-HSA-1643685","term_label":"Disease","supporting_discovery_ids":[3,18,20]}],"complexes":["PP2A holoenzyme (A-C-B56δ heterotrimer)","PPM1G-B56δ complex"],"partners":["PPP2CA","PPP2R1A","PPM1G","AKT","C/EBPΒ","PPIL1L LIPRIN-Α1 (PPFIA1)","NS5B"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"Q14738","full_name":"Serine/threonine-protein phosphatase 2A 56 kDa regulatory subunit delta isoform","aliases":["PP2A B subunit isoform B'-delta","PP2A B subunit isoform B56-delta","PP2A B subunit isoform PR61-delta","PP2A B subunit isoform R5-delta"],"length_aa":602,"mass_kda":70.0,"function":"The B regulatory subunit might modulate substrate selectivity and catalytic activity, and might also direct the localization of the catalytic enzyme to a particular subcellular compartment","subcellular_location":"Cytoplasm; Nucleus","url":"https://www.uniprot.org/uniprotkb/Q14738/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/PPP2R5D","classification":"Not Classified","n_dependent_lines":4,"n_total_lines":1208,"dependency_fraction":0.0033112582781456954},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[{"gene":"PPP2CA","stoichiometry":10.0},{"gene":"FKBP5","stoichiometry":0.2},{"gene":"PPP2CB","stoichiometry":0.2}],"url":"https://opencell.sf.czbiohub.org/search/PPP2R5D","total_profiled":1310},"omim":[{"mim_id":"621185","title":"HOUGE-JANSSENS SYNDROME 4; HJS4","url":"https://www.omim.org/entry/621185"},{"mim_id":"618354","title":"HOUGE-JANSSENS SYNDROME 3; HJS3","url":"https://www.omim.org/entry/618354"},{"mim_id":"616355","title":"HOUGE-JANSSENS SYNDROME 1; HJS1","url":"https://www.omim.org/entry/616355"},{"mim_id":"614766","title":"STRIATIN, CALMODULIN-BINDING PROTEIN 3; STRN3","url":"https://www.omim.org/entry/614766"},{"mim_id":"614765","title":"STRIATIN, CALMODULIN-BINDING PROTEIN; STRN","url":"https://www.omim.org/entry/614765"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Supported","locations":[{"location":"Cytosol","reliability":"Supported"},{"location":"Nucleoplasm","reliability":"Additional"}],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in 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The B56δ-containing PP2A complex dephosphorylates DARPP-32 at Thr-75, thereby mediating cAMP/PKA-dependent regulation of dopaminergic signaling in striatal neurons.\",\n      \"method\": \"In vitro kinase assay, site-directed mutagenesis (Ser-566 phosphorylation sites), cotransfection in HEK293 cells, striatal slice phosphorylation assays\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — in vitro reconstitution with mutagenesis, validated in vivo in striatal slices, multiple orthogonal methods in single rigorous study\",\n      \"pmids\": [\"17301223\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"PP2A/B56δ phosphatase complex dephosphorylates Cdc25 at T138 (Xenopus numbering), a site distinct from the inhibitory Ser287, thereby reducing Cdc25 affinity for 14-3-3 proteins and enabling 14-3-3 release to promote mitotic entry. DNA-responsive checkpoints activate PP2A/B56δ complexes, identifying B56δ as a central checkpoint effector.\",\n      \"method\": \"Xenopus cell-free extract and cell-based assays, immunoprecipitation, phosphorylation site mapping, dominant-negative and rescue experiments\",\n      \"journal\": \"Cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 / Strong — multiple orthogonal biochemical and cell-based assays, published in high-impact venue with rigorous controls\",\n      \"pmids\": [\"17110335\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"In Ppp2r5d knockout mice, tau becomes progressively hyperphosphorylated at pathological epitopes in restricted brain areas. The mechanism is indirect: PP2A-B56δ (PP2A-T61δ) dephosphorylates p35 (the CDK5 activator), and its absence leads to p35 hyperphosphorylation and degradation, thereby reducing CDK5 activity. Loss of CDK5 activity results in decreased phosphorylation of GSK3β at Ser-9, increasing GSK3β activity and causing tau hyperphosphorylation.\",\n      \"method\": \"Ppp2r5d knockout mouse model, in vitro dephosphorylation assays, immunohistochemistry, kinase activity assays, behavioral testing\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 / Strong — genetic KO with defined biochemical mechanism, in vitro phosphatase assay, multiple substrates tested\",\n      \"pmids\": [\"21482799\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"De novo missense mutations in PPP2R5D within a conserved acidic loop render mutant B56δ deficient in binding the PP2A A and C subunits, uncoupling it from phosphatase activity. This dominant-negative effect results in hyperphosphorylation of the B56δ-regulated substrate GSK3β in cells overexpressing mutant subunits.\",\n      \"method\": \"Co-immunoprecipitation for A/C subunit binding, phosphatase activity assays, overexpression of mutant subunits in cells, GSK3β phosphorylation immunoblot\",\n      \"journal\": \"The Journal of clinical investigation\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — reciprocal Co-IP, activity assays, substrate phosphorylation readout, replicated across multiple mutations in 16 patients\",\n      \"pmids\": [\"26168268\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"B56δ is the specific PP2A regulatory subunit mediating dephosphorylation of Bcl-2 at Ser70. Peroxynitrite-mediated nitration of B56δ at Tyr289 inhibits recruitment of the PP2A catalytic core (A and C subunits) to the B56δ complex while preserving B56δ binding to phospho-Ser70-Bcl-2, thereby preventing Bcl-2 dephosphorylation and stabilizing its antiapoptotic activity.\",\n      \"method\": \"Genetic knockdown of SOD1, co-immunoprecipitation, site-specific mutagenesis of B56δ Tyr289, PP2A holoenzyme assembly assays, Bcl-2 phosphorylation immunoblot\",\n      \"journal\": \"Blood\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 / Strong — mutagenesis of nitration site, reciprocal Co-IP for holoenzyme assembly, validated in primary lymphoma cells\",\n      \"pmids\": [\"25082878\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"Protein kinase C phosphorylates a key regulatory site in B56δ, activating the B56δ-containing PP2A heterotrimer, which then dephosphorylates Ser40 of tyrosine hydroxylase. RNAi knockdown of B56δ in N27 cells increases dopamine synthesis, confirming that PP2A-B56δ-mediated TH dephosphorylation reduces TH activity.\",\n      \"method\": \"In vitro kinase assay, RNAi knockdown, dopamine synthesis measurement, phosphorylation assays in cell lines\",\n      \"journal\": \"PloS one\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — RNAi KD with functional readout (dopamine synthesis), kinase assay, but single lab\",\n      \"pmids\": [\"22046270\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"PKA phosphorylates PPP2R5D at Ser-53, Ser-68, Ser-81, and Ser-566 in vitro, but Ser-566 is the dominant in vivo phosphorylation site upon forskolin stimulation. Phosphorylation of PPP2R5D by PKA reduces the apparent Km of PP2A holoenzyme from 11.25 μM to 1.175 μM, increasing catalytic efficiency. Phosphorylation also decreases inhibitory Tyr-307 phosphorylation on the PP2A catalytic C subunit.\",\n      \"method\": \"In vitro kinase assay, site-directed mutagenesis of phosphorylation sites, kinetic analysis in HEK293 cells with forskolin, phospho-immunoblot\",\n      \"journal\": \"BMB reports\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 / Weak — in vitro kinase assay and kinetics with mutagenesis, but single lab and limited validation\",\n      \"pmids\": [\"20423611\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"β-Adrenergic receptor stimulation induces PKA-mediated phosphorylation of B56δ at Ser573 in adult rat ventricular cardiomyocytes. A non-phosphorylatable S573A mutant B56δ fails to increase PP2A catalytic activity in response to isoprenaline, demonstrating that Ser573 phosphorylation is required for β-AR-stimulated PP2A activation in cardiomyocytes.\",\n      \"method\": \"Phosphate-affinity SDS-PAGE, adenoviral transduction with WT and S573A mutant B56δ-GFP, co-immunoprecipitation with A/C subunits, PP2A activity assay, phosphoproteomics immunoblotting in ARVMs\",\n      \"journal\": \"Journal of molecular and cellular cardiology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — mutagenesis in relevant cell type, co-IP, activity assay, single lab with two orthogonal methods\",\n      \"pmids\": [\"29294329\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"PPM1G (a PPM-family phosphatase) forms a novel holoenzyme complex with B56δ (PPP2R5D). B56δ promotes relocalization of PPM1G from the nucleus to the cytoplasm. The PPM1G-B56δ complex dephosphorylates α-catenin at Ser641 in the cytoplasm, which is required for proper adherens junction assembly and prevention of aberrant cell migration.\",\n      \"method\": \"Co-immunoprecipitation, subcellular fractionation/live imaging for relocalization, in vitro phosphatase assay on α-catenin, knockdown/rescue experiments for cell migration and junction assembly\",\n      \"journal\": \"EMBO reports\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 / Strong — Co-IP of novel complex, substrate identified with in vitro dephosphorylation, functional consequence (adherens junctions, migration) validated\",\n      \"pmids\": [\"31432583\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"B56δ-containing PP2A directly binds C/EBPβ and dephosphorylates it during early adipogenesis. This dephosphorylation is required for C/EBPβ degradation, which allows subsequent expression of PPARγ and C/EBPα and completion of adipogenesis.\",\n      \"method\": \"Co-immunoprecipitation of B56δ with C/EBPβ, okadaic acid inhibition, knockdown of specific B56 subunits, adipogenesis induction assay, immunoblotting for adipogenic transcription factors\",\n      \"journal\": \"Biochimica et biophysica acta\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2-3 / Moderate — direct binding shown by Co-IP, functional knockdown with defined transcription factor readout, single lab\",\n      \"pmids\": [\"25152162\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"B56δ-containing PP2A dephosphorylates eNOS at Ser116. Aphidicolin-induced DNA damage increases B56δ-Ser566 phosphorylation, activating PP2A-B56δ, which dephosphorylates eNOS-Ser116 and contributes to NO release in endothelial cells. Dominant-negative B56δ blocks both Ser116 dephosphorylation and NO production.\",\n      \"method\": \"Dominant-negative B56δ overexpression, okadaic acid inhibition, phospho-immunoblot for eNOS-Ser116 and B56δ-Ser566, NO measurement in bovine aortic endothelial cells\",\n      \"journal\": \"Nitric oxide : biology and chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2-3 / Moderate — dominant-negative mutation with two phosphorylation site readouts and functional NO assay, single lab\",\n      \"pmids\": [\"26255574\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"A CRISPR-base-edited E420K heterozygous variant of PPP2R5D in HEK293 cells reveals a direct interaction between PPP2R5D and AKT, leading to constitutively active AKT-mTOR signaling, increased cell size, and uncoordinated cellular growth. Rapamycin reduces cell size and RPS6 hyperphosphorylation in E420K variant cells.\",\n      \"method\": \"CRISPR single-base editing to introduce E420K variant, quantitative phosphoproteomics (TMT-LC-MS3), co-immunoprecipitation of PPP2R5D-AKT, rapamycin treatment, cell size measurement\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 / Strong — precise endogenous variant introduced by CRISPR, unbiased phosphoproteomics plus orthogonal Co-IP, functional cell size readout, single lab with multiple methods\",\n      \"pmids\": [\"33482199\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"Loss of PP2A-B56δ in knockout mice leads to spontaneous hepatocellular carcinoma, associated with c-Myc Ser62 hyperphosphorylation and GSK3β Ser9 hyperphosphorylation in non-cancerous B56δ-null livers, indicating that B56δ-driven GSK3β inactivation controls c-Myc activity as a tumor suppressive mechanism.\",\n      \"method\": \"Ppp2r5d knockout mouse model, targeted immunoblotting, immunohistochemistry, RNA sequencing, phospho-specific antibodies for c-Myc Ser62 and GSK3β Ser9\",\n      \"journal\": \"Oncogene\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — genetic KO with phosphoprotein readouts and transcriptomic support, single lab\",\n      \"pmids\": [\"28967903\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"PPP2R5D interacts specifically with HCV NS5B RNA-dependent RNA polymerase (but not HCV Core or NS3), colocalizes with NS5B in the endoplasmic reticulum, and is required for HCV replication. Knockout of PPP2R5D abolishes HCV infection in Huh7.5 cells, and re-expression restores infection.\",\n      \"method\": \"Co-immunoprecipitation, colocalization by imaging, PPP2R5D knockout and knockdown with complementation, HCV replicon replication assay\",\n      \"journal\": \"Virology journal\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2-3 / Moderate — reciprocal Co-IP plus KO/complementation with viral replication readout, single lab\",\n      \"pmids\": [\"35836293\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"Cryo-EM structures of the PP2A-B56δ holoenzyme reveal that long intrinsically disordered regions (IDRs) at B56δ N- and C-termini fold against each other and the holoenzyme core, forming a closed latent conformation with dual autoinhibition of the phosphatase active site and the substrate-binding groove. This interface spans >190 Å, harbors activation phosphorylation sites and essentially all ID-associated mutation residues. ID mutations increase holoenzyme activity and alter phosphorylation rates; severe variants significantly increase mitotic duration and error rates.\",\n      \"method\": \"Single-particle cryo-EM structure determination, in vitro phosphorylation assays, mitotic duration and error rate measurements in cells with disease variants\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — cryo-EM structural determination combined with functional phosphorylation and cell-biological assays for disease mutations\",\n      \"pmids\": [\"38150499\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"Quantum mechanical calculations on the PP2A(PPP2R5D)/phosphoserine system indicate that bidentate Arg89-substrate binding is critical for optimal catalytic function, yielding ΔH‡ = +15.5 kcal/mol vs +18.8 kcal/mol when Arg89 is engaged in a salt bridge with B56δ Glu198. The pathogenic E198K mutation replaces the acidic Glu198 with a positively charged Lys, disrupting this salt bridge and altering the catalytic mechanism.\",\n      \"method\": \"Quantum-based hybrid ONIOM(UB3LYP/6-31G(d):UPM7) computational modeling of 39-residue active-site models\",\n      \"journal\": \"Frontiers in cell and developmental biology\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 4 / Weak — computational modeling only, no experimental validation in this study\",\n      \"pmids\": [\"37377738\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"PPP2R5D/PP2A-B56δ interacts with liprin-α1 through a canonical short linear interaction motif (SLiM) in liprin-α1's N-terminal dimerization domain. Loss of this interaction (SLiM mutation or PPP2R5D KO) results in increased liprin-α1 phosphorylation at Ser763 and promotes liprin-α1 liquid-liquid phase separation (LLPS). The E420K disease variant of PPP2R5D compromises suppression of liprin-α1 LLPS.\",\n      \"method\": \"MS-based interactomics, co-immunoprecipitation, SLiM mutagenesis, PPP2R5D KO cells, GFP-liprin-α1 LLPS imaging, phospho-mass spectrometry\",\n      \"journal\": \"bioRxiv\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2-3 / Moderate — MS interactomics with SLiM mutagenesis, KO cell validation, LLPS imaging; preprint\",\n      \"pmids\": [\"38948786\"],\n      \"is_preprint\": true\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"PPP2R5D-PP2A holoenzyme inhibits liprin-α1 LLPS by dephosphorylating liprin-α1 at multiple Ser/Thr sites including Ser763. The phospho-mimetic S763E mutant is sufficient to drive liprin-α1 LLPS. The E420K PPP2R5D disease variant increases liprin-α1 Ser763 phosphorylation and promotes LLPS. The interaction also promotes liprin-α1/β1 heterodimerization, which opposes LLPS.\",\n      \"method\": \"Mass spectrometry phospho-analysis, phospho-specific antibody validation, SLiM mutagenesis, PPP2R5D KO and E420K knock-in cells, LLPS imaging, liprin-α1/β1 co-immunoprecipitation\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 / Strong — multiple orthogonal methods (phospho-MS with novel antibody validation, KO and knock-in cells, SLiM mutagenesis, LLPS functional readout) in single rigorous study\",\n      \"pmids\": [\"40484382\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"Functional characterization of PPP2R5D missense variants shows that pathogenic variants cause impaired PP2A A/C-subunit binding, decreased SLiM-dependent substrate binding, or both. The most severe clinical phenotypes associate with variants that completely lose one of these binding properties while retaining the other, supporting a dominant-negative disease mechanism. The p.Glu198Lys variant shows the highest C-binding defect.\",\n      \"method\": \"Co-immunoprecipitation for A/C subunit binding, SLiM-dependent substrate binding assays, correlation with clinical phenotype severity in 76 patients\",\n      \"journal\": \"Journal of medical genetics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — Co-IP binding assays for multiple variants with phenotypic correlation, single lab\",\n      \"pmids\": [\"36216457\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"Double knockout of Ppp2r5d (B56δ) and Ppp2r5c (B56γ) in mice causes arrest of fetal development around E12 and results in a single cardiac outflow vessel instead of separate aorta and pulmonary artery, demonstrating a genetic interaction between B56δ and B56γ that is required for heart development. Neither single knockout alone is lethal.\",\n      \"method\": \"CRISPR/Cas9n knockout mouse generation, genetic epistasis (double KO), embryonic lethal phenotype analysis, cardiac anatomy assessment\",\n      \"journal\": \"FASEB bioAdvances\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — genetic epistasis in mouse KO model with defined developmental phenotype, single lab\",\n      \"pmids\": [\"35415460\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"Quantitative phosphoproteomics of CRISPR-edited E198K and E420K PPP2R5D variant HEK293 cells reveals hyperphosphorylation of RPS6 as a shared signaling alteration, mediated through converging mTORC1/p70S6K activation. E420K shows AKT-dependent mTORC1 activation while E198K shows AKT-independent ERK-dependent activation. Rapamycin and the S6K inhibitor LY2584702 suppress RPS6 hyperphosphorylation.\",\n      \"method\": \"CRISPR-PRIME editing for E198K variant, global quantitative proteomics and phosphoproteomics, rapamycin and kinase inhibitor treatments, immunoblotting\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — global phosphoproteomics on endogenous CRISPR-edited variants, pharmacological validation, single lab\",\n      \"pmids\": [\"37572851\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"PPP2R5D encodes B56δ, a regulatory subunit that assembles with PP2A scaffold (A) and catalytic (C) subunits to form a heterotrimer whose substrate specificity, subcellular localization, and activity are governed by B56δ; PKA and PKC phosphorylate B56δ at Ser566/Ser573 to increase PP2A catalytic efficiency (lowering Km), while peroxynitrite nitration of B56δ Tyr289 disassembles the holoenzyme; structurally, long intrinsically disordered arms of B56δ fold against the holoenzyme core to create dual autoinhibition that is relieved by activating phosphorylation; established substrates dephosphorylated by PP2A-B56δ include DARPP-32 (Thr75), Cdc25 (T138), p35/CDK5 activator, GSK3β (controlling tau phosphorylation and c-Myc stability), Bcl-2 (Ser70), tyrosine hydroxylase (Ser40), C/EBPβ (early adipogenesis), eNOS (Ser116), α-catenin (Ser641 via PPM1G-B56δ), AKT, and liprin-α1 (Ser763); disease-causing de novo missense mutations disrupt A/C-subunit binding or SLiM-dependent substrate recruitment in a dominant-negative manner, leading to hyperactivation of AKT-mTOR-RPS6 and GSK3β-c-Myc signaling cascades underlying intellectual disability, macrocephaly, and tumorigenesis.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"PPP2R5D encodes B56\\u03b4, a substrate-specifying regulatory subunit that assembles with the PP2A scaffold (A) and catalytic (C) subunits into a heterotrimeric holoenzyme directing protein dephosphorylation across neuronal signaling, cell-cycle control, and developmental programs [#0, #3]. Holoenzyme activity is gated by upstream phosphorylation: PKA and PKC phosphorylate B56\\u03b4 at Ser566/Ser573 to raise catalytic efficiency by lowering the apparent Km and reducing inhibitory phosphorylation of the C subunit [#0, #6, #7], a regulatory logic explained structurally by intrinsically disordered B56\\u03b4 termini that fold against the holoenzyme core to impose dual autoinhibition of both the active site and the substrate-binding groove, relieved by these activating phosphorylation events [#14]. Through this holoenzyme, B56\\u03b4 dephosphorylates a wide substrate set with distinct physiological outputs\\u2014DARPP-32 Thr75 in dopaminergic signaling [#0], Cdc25 to license mitotic entry [#1], the CDK5 activator p35 governing GSK3\\u03b2-dependent tau phosphorylation [#2], tyrosine hydroxylase Ser40 controlling dopamine synthesis [#5], Bcl-2 Ser70 [#4], C/EBP\\u03b2 during adipogenesis [#9], eNOS Ser116 [#10], and liprin-\\u03b11 Ser763, where dephosphorylation suppresses liquid-liquid phase separation [#16, #17]. B56\\u03b4 also nucleates non-canonical complexes, partnering with PPM1G to relocalize it and dephosphorylate \\u03b1-catenin Ser641 at adherens junctions [#8]. Loss of B56\\u03b4 inactivates GSK3\\u03b2 control of c-Myc, driving spontaneous hepatocellular carcinoma in knockout mice [#12], and B56\\u03b4 is genetically required with B56\\u03b3 for cardiac outflow tract development [#19]. De novo missense mutations in PPP2R5D cause an intellectual disability and macrocephaly syndrome through a dominant-negative mechanism, impairing A/C-subunit binding and/or SLiM-dependent substrate recruitment and producing hyperactivation of AKT-mTOR-RPS6 and GSK3\\u03b2-c-Myc signaling [#3, #11, #18, #20].\",\n  \"teleology\": [\n    {\n      \"year\": 2006,\n      \"claim\": \"Established B56\\u03b4-PP2A as a cell-cycle checkpoint effector by identifying how it relieves 14-3-3 sequestration of Cdc25 to control mitotic entry.\",\n      \"evidence\": \"Xenopus cell-free extract and cell-based assays with phosphosite mapping, dominant-negative and rescue experiments\",\n      \"pmids\": [\"17110335\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not resolve how DNA-damage checkpoints physically activate the B56\\u03b4 holoenzyme\", \"Human in vivo relevance not addressed\"]\n    },\n    {\n      \"year\": 2007,\n      \"claim\": \"Showed that B56\\u03b4 phosphorylation is an activating switch, linking PKA phosphorylation of Ser566 to increased phosphatase activity and DARPP-32 dephosphorylation in neurons.\",\n      \"evidence\": \"In vitro kinase assay, site-directed mutagenesis, HEK293 cotransfection, striatal slice phosphorylation assays\",\n      \"pmids\": [\"17301223\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural basis of phosphorylation-induced activation unknown at this stage\", \"Full substrate repertoire untested\"]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"Quantified the activating mechanism, showing PKA phosphorylation of Ser566 lowers holoenzyme Km roughly tenfold and decreases inhibitory C-subunit Tyr307 phosphorylation.\",\n      \"evidence\": \"In vitro kinase assay, mutagenesis, kinetic analysis with forskolin in HEK293 cells\",\n      \"pmids\": [\"20423611\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single lab, limited validation\", \"Roles of additional sites Ser53/68/81 in vivo not resolved\"]\n    },\n    {\n      \"year\": 2011,\n      \"claim\": \"Connected B56\\u03b4 loss to neurodegenerative tau pathology via an indirect p35/CDK5/GSK3\\u03b2 cascade, and to dopamine synthesis control via tyrosine hydroxylase Ser40.\",\n      \"evidence\": \"Ppp2r5d knockout mouse with in vitro dephosphorylation assays; PKC kinase assay with RNAi and dopamine measurement in N27 cells\",\n      \"pmids\": [\"21482799\", \"22046270\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Direct versus indirect substrate relationships partly inferred\", \"TH study from single lab\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Revealed that post-translational modification can selectively disassemble the holoenzyme, with peroxynitrite nitration of B56\\u03b4 Tyr289 blocking A/C recruitment while sparing substrate binding to stabilize Bcl-2.\",\n      \"evidence\": \"SOD1 knockdown, Tyr289 mutagenesis, holoenzyme assembly Co-IP, Bcl-2 phospho-immunoblot in lymphoma cells\",\n      \"pmids\": [\"25082878\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Generality of nitration regulation across tissues unknown\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"Extended the substrate landscape to metabolic and vascular programs through C/EBP\\u03b2 dephosphorylation in adipogenesis and eNOS Ser116 dephosphorylation in endothelial NO release.\",\n      \"evidence\": \"Co-IP, knockdown, adipogenesis assays; dominant-negative B56\\u03b4, phospho-immunoblot and NO measurement in endothelial cells\",\n      \"pmids\": [\"25152162\", \"26255574\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Both single-lab studies\", \"Direct holoenzyme-substrate engagement not structurally defined\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"Defined the dominant-negative disease mechanism, showing de novo mutations in a conserved acidic loop uncouple B56\\u03b4 from A/C binding and cause GSK3\\u03b2 hyperphosphorylation.\",\n      \"evidence\": \"Reciprocal Co-IP, phosphatase activity assays, mutant overexpression, GSK3\\u03b2 immunoblot across 16 patients\",\n      \"pmids\": [\"26168268\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Endogenous-level consequences not yet tested\", \"Downstream signaling network not mapped\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"Tied B56\\u03b4 to tumor suppression in vivo, with knockout livers developing hepatocellular carcinoma alongside GSK3\\u03b2 and c-Myc Ser62 hyperphosphorylation; also confirmed Ser573 as a \\u03b2-adrenergic activation site in cardiomyocytes.\",\n      \"evidence\": \"Ppp2r5d KO mouse with phospho-immunoblot, IHC, RNA-seq; phosphate-affinity gels and S573A mutant in adult rat ventricular myocytes\",\n      \"pmids\": [\"28967903\", \"29294329\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single-lab studies\", \"Causal chain from GSK3\\u03b2 to tumorigenesis not fully dissected\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"Uncovered a non-canonical role, in which B56\\u03b4 partners with PPM1G to relocalize it and dephosphorylate \\u03b1-catenin Ser641 for adherens junction assembly.\",\n      \"evidence\": \"Co-IP, subcellular fractionation/live imaging, in vitro phosphatase assay, knockdown/rescue migration assays\",\n      \"pmids\": [\"31432583\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Stoichiometry and structural basis of the PPM1G-B56\\u03b4 complex unknown\", \"Whether PP2A C subunit participates unclear\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Resolved how disease variants drive growth signaling using endogenous CRISPR-edited cells, linking E420K B56\\u03b4 to an AKT interaction and constitutive AKT-mTOR-RPS6 activation; double KO with B56\\u03b3 showed an essential role in heart development.\",\n      \"evidence\": \"CRISPR base editing of E420K, phosphoproteomics, PPP2R5D-AKT Co-IP, rapamycin rescue; CRISPR/Cas9n double-KO mouse with cardiac anatomy analysis\",\n      \"pmids\": [\"33482199\", \"35415460\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether AKT is a direct dephosphorylation substrate not fully established\", \"Redundancy logic between B56\\u03b4 and B56\\u03b3 not molecularly defined\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Systematically mapped variant biochemistry, separating A/C-binding defects from SLiM-dependent substrate-binding defects and correlating them with phenotype severity; also identified a host-factor role in HCV replication.\",\n      \"evidence\": \"Co-IP binding assays across variants with clinical correlation in 76 patients; Co-IP, colocalization and KO/complementation HCV replication assays\",\n      \"pmids\": [\"36216457\", \"35836293\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Mechanism by which NS5B exploits B56\\u03b4 unknown\", \"Binding-assay severity correlation is associative\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Provided the structural and mechanistic basis of regulation, showing disordered B56\\u03b4 arms create dual autoinhibition spanning activation sites and disease residues, with variants increasing activity and mitotic errors; computational modeling implicated an Arg89-Glu198 salt bridge disrupted by E198K.\",\n      \"evidence\": \"Cryo-EM structure determination with in vitro phosphorylation and mitotic error assays; QM/ONIOM active-site modeling\",\n      \"pmids\": [\"38150499\", \"37377738\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"QM modeling lacks experimental validation (Low confidence)\", \"How specific variants quantitatively shift substrate selectivity not fully resolved\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Established liprin-\\u03b11 as a SLiM-recruited substrate whose Ser763 dephosphorylation suppresses liquid-liquid phase separation and promotes heterodimerization, with E420K B56\\u03b4 failing to restrain this LLPS.\",\n      \"evidence\": \"MS interactomics, phospho-MS with antibody validation, SLiM mutagenesis, KO and E420K knock-in cells, LLPS imaging, Co-IP\",\n      \"pmids\": [\"40484382\", \"38948786\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Physiological consequence of liprin-\\u03b11 LLPS dysregulation in patient neurons not shown\", \"Whether LLPS control generalizes to other B56\\u03b4 substrates unknown\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"It remains unresolved how individual disease variants convert B56\\u03b4's dual-autoinhibited holoenzyme into specific gain- versus loss-of-function outputs on distinct substrates, and how these reconcile into the unified ID-macrocephaly-tumor phenotype.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No structure of disease-variant holoenzyme bound to substrate\", \"Tissue-specific substrate selectivity not mapped\", \"Direct versus indirect status of several substrates (AKT) unconfirmed\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0140096\", \"supporting_discovery_ids\": [0, 1, 2, 4, 8, 9, 10, 17]},\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [0, 3, 14, 18]},\n      {\"term_id\": \"GO:0060090\", \"supporting_discovery_ids\": [8, 16, 17]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005829\", \"supporting_discovery_ids\": [8]},\n      {\"term_id\": \"GO:0005634\", \"supporting_discovery_ids\": [8]},\n      {\"term_id\": \"GO:0005783\", \"supporting_discovery_ids\": [13]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-1640170\", \"supporting_discovery_ids\": [1, 14]},\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [0, 11, 20]},\n      {\"term_id\": \"R-HSA-392499\", \"supporting_discovery_ids\": [0, 6, 7]},\n      {\"term_id\": \"R-HSA-1643685\", \"supporting_discovery_ids\": [3, 18, 20]}\n    ],\n    \"complexes\": [\"PP2A holoenzyme (A-C-B56\\u03b4 heterotrimer)\", \"PPM1G-B56\\u03b4 complex\"],\n    \"partners\": [\"PPP2CA\", \"PPP2R1A\", \"PPM1G\", \"AKT\", \"C/EBP\\u03b2\", \"PPIL1L liprin-\\u03b11 (PPFIA1)\", \"NS5B\"],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":6,"faith_total":6,"faith_pct":100.0}}